[Compute Separation] M5c: Integration tests

[brettsam]

End-to-end validation of the external worker model.

Dependencies: M4 (#11647), M5a (#11649)

Acceptance Criteria

• Test: external worker connects → functions execute → responses correct
• Test: worker disconnects mid-invocation → invocation fails gracefully with clear error
• Test: worker disconnects and reconnects (new workerId) → new invocations succeed
• Test: placeholder → specialization → worker connects → functions execute
• Test: host-managed mode regression — all existing E2E tests pass with feature disabled
• Tests added to CI pipeline (integration test group)